1. Field of the Invention
The present invention relates to a radio communication apparatus and, more particularly, to an apparatus where the output level of a transmission signal is adjustable.
2. Description of the Related Art
In a duplex mobile telephone system inclusive of portable telephones and car telephones such as a cellular phone system, a base station is capable of controlling or designating the transmission output of a mobile station. This is to accomplish the purpose of suppressing the transmission output of a mobile station to the minimum value required for the base station, so as to increase the number of mobile stations, which can utilize the system, by using the same channel even in a cell (service area) of the other base station.
In the TACS (Total Access Communication System), the control or designation of the transmission output of a mobile station is realized by transmission of a code termed MAC (Mobile Attenuation Code) from the base station to the mobile station. The relationship between the MAC and the transmission output is shown in FIG. 1. More specifically, the MAC is data of 3 bits, and therefore it is possible to designate the transmission output in the maximum of 8 steps. The transmission output is classified into three classes from 1 to 3; i.e., class 1 is assigned to car telephones or the like, class 2 to car telephones and shoulder-type mobile telephones, and class 3 to handy-type portable telephones, respectively.
Accordingly, in a car telephone of the class 1, its transmission output is controlled or designated in 8 steps at an interval of 4 dB. Meanwhile in a portable telephone of the class 3, its maximum output is predetermined to be less than half the value in the car telephone of the class 1 in consideration of the service life of a battery used as a power supply.
Further, although some errors are permitted legally in such transmission outputs, the allowable error range of each transmission output is set to be relatively wide as shown in FIG. 1.
A method of changing the transmission output in accordance with the MAC will now be described below with reference to FIG. 2. FIG. 2 is a block diagram of a radio communication apparatus which is a premise of the present invention.
In FIG. 2, there is shown a mouthpiece 11, an audio circuit 12, a transmitting circuit 13, a power amplifier 14, a duplexer 15, a transmitting/receiving antenna 16, a receiving circuit 17, and an earpiece 18.
When a sound signal from the mouthpiece 11 is supplied via the audio circuit 12 to the transmitting circuit 13, it is converted into an up-channel transmission signal (FM signal) S13. This transmission signal is amplified in the power amplifier 14. This amplified signal S13 is supplied via the duplexer 15 to the antenna 16, from which the signal S13 is transmitted to the base station.
Meanwhile a down-channel FM signal (transmission signal) from the base station is received at the antenna 16. The signal thus received is supplied via the duplexer 15 to the receiving circuit 17, so that the sound signal from the talker is extracted. Thereafter the sound signal thus extracted is supplied via the audio circuit 12 to the earpiece 18.
Denoted by 21 is a system controller which consists of a microcomputer. The transmitting circuit 13 and the receiving circuit 17 are connected to each other via a modem circuit 22, whereby a predetermined command signal or data such as MAC can be accessed between the system controller 21 and the base station. Further, the audio circuit 12, the transmitting circuit 13 and the receiving circuit 17 are controlled by the system controller 21 in response to the command signal or the data received from the base station. Denoted by 23 is an operation key such as a dial key or a talk key.
Reference numeral 30 denotes an APC (Auto Power Control) circuit for controlling the transmission output in accordance with the MAC. Data D31 is obtained from an output port of the system controller 21 having a latch function. The data D31 is used for designating the transmission output and has, e.g., complementary correlation to the MAC. The data D31 is supplied to a D-A converter 31, where a digital-to-analog conversion is executed to convert the data D31 into an analog voltage V31. The voltage V31 thus obtained through the digital-to-analog conversion is supplied as a reference voltage to a voltage comparator 32. Further the transmission signal from the power amplifier 14 is partially supplied to a detector 33, which then outputs a DC-voltage V33 of a level corresponding to the level or amplitude of the transmission signal S13, and then the voltage V33 is supplied to the comparator 32.
The emitter-collector junction of a transistor 35 is connected in series between a power supply terminal 34 and a power line of the amplifier 14, and a comparison output voltage V32 obtained from the comparator 32 is supplied to the base of the transistor 35 as a control signal for the transistor 35.
Accordingly, when the comparison output voltage V32 is changed, the operating voltage supplied to the amplifier 14 is also changed in correspondence to such a voltage change. And due to the change caused in the operating voltage, there occurs a change in the level of the transmission signal S13 to be outputted from the amplifier 14. In this stage, feedback is being executed by the comparator 32 in such a manner as to render V32=V31, so that the level (V33) of the transmission signal S13 outputted from the amplifier 14 becomes equal to the level of the voltage V31.
Therefore, the level of the transmission signal S13 can be controlled in the system controller 21 by controlling the value of the data D31, whereby the base station is enabled to control or designate the transmission output of the mobile station in accordance with the MAC.
In the mobile telephone system, as described above, the transmission output of a mobile station is suppressed to the necessary minimum value. Practically, however, fine designation of the transmission output is not executed in a base station and the maximum output is permitted in most cases. In other words, the current consumption in a mobile station becomes large.
In this case, if the mobile station is a car telephone of the class 1, substantially no problem exists with respect to the current consumption since the operating power is supplied from a car battery. However, in a portable telephone of the class 3 which uses the battery incorporated therein for its operation, the service life of the battery is shortened if the current consumption is large, hence shortening the effective time for communication.
Even if the base station executes fine designation of the transmission output, the allowable range of the transmission output is as wide as +2 dB to -4 dB (or -2 dB). Therefore, in the stage of designing or manufacturing a portable telephone, the current consumption thereof varies widely depending on whether the transmission output is set to a rated value of +2 dB or -2 dB, whereby the service life of the battery is considerably changed.
Furthermore, when a portable telephone and a base station are connected to each other at the time of an originating call or a terminating call, it is necessary, for achieving enhanced connection, to perform handshake of the data required for control of the connection at the highest possible level. However, if such operation is performed, the transmission output during a call is increased within the allowable range to consequently cause wasteful consumption of the battery.